PROJECT SUMMARY/ ABSTRACT GABA-containing Inhibitory interneurons are <20% of all cortical neurons, yet are critically important for proper cortical development and function. Dysfunction of inhibitory cortical interneurons is implicated in a wide range of mental and neurological disorders. Inhibitory interneurons comprise several subclasses which are distinguishable by their neurochemical content, morphology, electrophysiology and synaptic connectivity. A prominent subclass are somatostatin-containing interneurons, which preferentially target distal dendrites of excitatory neurons. What functions this distal inhibition performs in cortical computations, and how these interneurons contribute to sensation, perception and behavior, is still not fully understood. The PI has previously shown that somatostatin interneurons comprise (at least) two broad subsets which differ in their main axonal targets: those targeting most heavily layer 4, the layer receiving ?bottom-up? sensory information, and those targeting layer 1, the layer receiving ?top-down? contextual information from higher-order brain regions. The objective of the proposed research is to develop novel genetic approaches to target these distinct subsets and to shed light on their roles in cortical computations underlying behavior and learning. The proposed research will test the hypothesis that layer 4- and layer 1-targeting somatostatin interneurons have diverged to modulate and constrain the two major input systems of the cortex, and that during associative learning these two systems work in a push-pull fashion, to shift the balance between these two pathways in favor of top-down inputs. The proposed study will employ a novel combinatorial genetic approach with newly developed intersectional viral vectors, to separately target each of the two subsets for electrophysiological recording, two-photon calcium imaging and light-induced activation. Using these tools, the applicants will examine the detailed pattern of synaptic connections between these interneuron subsets and excitatory neurons, and will monitor changes in these two inhibitory systems as the animal learns to associate two stimuli. Achieving the aims of the proposal would be a major advance in our understanding of how cortical circuits are organized, how they evolved and how they are rewired during learning.